Gasification is a commercially proven technology that efficiently converts petroleum coke, coal, heavy oil streams and even natural gas into synthesis gas through a non-combustion, partial oxidation reaction.
Synthesis gas can be used as a cleaner-burning fuel for gas turbines, such as in an IGCC system, to produce electricity and/or steam. In addition to generating electricity and/or steam, synthesis gas can also be used to generate hydrogen for use in heavy oil upgraders and hydroprocessing units in a refinery. Further, synthesis gas components such hydrogen, carbon monoxide, carbon dioxide and nitrogen constitute the basic building blocks of may valuable chemical products such as oxo-alcohols, methanol, ammonia, Fischer-Tropsch liquids, e.g., ultra-low sulfur diesel, plastics and chemical fertilizers.
The raw synthesis gas stream is typically subjected to one or more separation processes depending on the type of products to be produced by the IGCC system. One gas separation process which is generally necessary in an IGCC system is the selective removal of sulfur compounds such as carbonyl sulfide and hydrogen sulfide from the raw synthesis gas.
The removal of such sulfur-containing compounds is desirable for many reasons, depending in part upon the intended use of the final gas product. Since a large percentage of the produced synthesis gas is typically used as fuel in a gas turbine, the presence of sulfur-containing compounds is generally objectionable because of one or more concerns such as, involving: safety, corrosion and pollution as well as the unpleasant odor commonly associated with the sulfur-containing compounds. Additionally, such sulfur-containing compounds can have a deleterious effect on downstream equipment and systems used for the production of hydrogen and other chemicals.
One separation technique used to remove sulfur-containing compounds from the raw synthesis gas stream involves contacting the raw synthesis gas stream with a solvent to selectively absorb the sulfur-containing compounds. However, as the liquid solvent selectively absorbs sulfur compounds from the raw synthesis gas stream, it also co-absorbs carbon dioxide. The co-absorbed carbon dioxide, if not removed upstream of a sulfur recovery unit such as, for example, a Claus unit, can negatively impact the capital and operating costs of the sulfur recovery unit. Advantageously, recovery of the co-absorbed carbon dioxide results in additional power generation.
Using nitrogen to strip co-absorbed carbon dioxide from the liquid solvent presents advantages over using treated, i.e., sulfur-free, synthesis gas. For example, hydrogen losses from the sulfide absorption unit are minimized if nitrogen is used.
Although nitrogen is readily available from an air separation unit supplying the oxygen required for gasification, the purity of the nitrogen stream used for stripping typically needs to be upgraded from about 97% to 99.9% by volume. Further, depending on where the nitrogen is being utilized within the sulfide absorption unit, compression may be required. As will be appreciated by those skilled in the art, the production or even procurement of the high-purity, compressed nitrogen which is generally required to effectively strip carbon dioxide from the liquid solvent can detrimentally add to the expense associated with the production of desired product gases.
Additional processes for removing sulfur-containing compounds and/or carbon dioxide from gas streams are described in U.S. Pat. No. 3,362,133 to Kutsher et al., U.S. Pat. No. 4,330,305 to Kuessner et al., U.S. Pat. No. 5,861,051 to Critchfield et al., and U.S. Pat. No. 6,203,599 to Schubert et al.
In addition to a sulfide absorption unit, a synthesis gas processing block typically also includes one or more purification and/or recovery units such as, for example, a sulfur recovery unit and hydrogen separation and purification units. Such purification and/or recovery units generally produce byproduct or waste streams which may contain residual levels of the components desirably isolated by the purification and/or recovery units such as, for example, carbon dioxide, hydrogen and sulfur-containing compounds. Typically, such byproduct or waste streams, depending upon chemical composition, may be recycled into the synthesis gas processing block, off-gassed or combusted as fuel. The fate of such byproduct or waste streams may largely depend upon the costs, both monetary and in terms of energy expenditures, needed to recover the residual levels of desirable compounds.
Thus, there is a need and a demand for processing schemes that improve the efficiency and economics of separating at least hydrogen sulfide and carbon dioxide from a fluid stream.
There is a further need and a demand for processes for separating at least hydrogen sulfide and carbon dioxide from a fluid stream having reduced dependence upon external gas inputs.
There is an additional need and a demand for processes for the separation of hydrogen sulfide and carbon dioxide from a fluid stream which are effective to result in an increased recovery of desired and/or beneficial gases from process byproduct or waste streams.